Distribution facilities such as for gas, petroleum and the like normally utilize cathodically protected steel piping laid in the soil. Cathodic protection is a means of protecting buried coated steel pipe from corrosion by maintaining it at a small voltage (usually less than 1V negative) relative to the surrounding soil.
Such cathodically protected coated steel pipe suffers from this protection being lost through random, unpredictable and numerous grounds. A ground is any direct contact with other structures or with the soil itself, thereby shorting out the coating, and which drains away the protective electricity thus exposing the protected material to corrosion. Gas, oil and other hazardous fluid lines are particularly vulnerable because corrosion occurs in small areas which results in leaks which are much more dangerous and costly than the actual material damage to the conduit.
Every such ground starts the entire connected section of pipe, large or small, on which it occurs, on a demonstrable "corrosion leak curve" which, rising on a log-log progression will, if unchecked, destroy the pipe past repair in a predictable number of years. Although this curve varies, there are few, if any, cases where such corrosion damage is acceptable for the life of the type of conduit.
It is therefore usual for a gas or oil distribution utility or the like to practice corrosion control by endeavoring to check the maintenance of the cathodic protection on a regular basis.
This means, in practice, endeavoring to locate and repair ground faults by means of instrumentation presently available. As an example, cathodic protection maintenance at the Greater Winnipeg Gas Company, Winnipeg, Manitoba, Canada, means, in practice, locating and repairing grounds in underground piping, which may number between 700 and 900 a year on 1,200 miles of distribution mains even although these distribution mains are provided with supposedly ground-avoiding designs i.e. coating on the steel pipe, backed up with cathodic protection.
Prior to 1960, the main method for locating grounds was to test the soil voltage gradients. Then an AC caliper device was offered to check services in series down a given street to locate any that were conveying an AC hum.
Around 1960, the Nilsson pipe and cable trace (Wahlquist patent) was used, the signal of which was ground seeking and could be so traced to the point of the nearest ground. Up until the present time, this has been the most efficient method of locating such grounds. As mentioned previously, it is based on the Wahlquist patent which is an adaption of the Pearson method for tracing soil currents. A ground seeking audio frequency signal is injected into one point of the pipe and then the course of the signal is followed with a receiver. The principal limitation is the frequent ambiguity of the operators indication. Furthermore, the receiver is subject to various electronic noises so that in general, it has been found to be unsatisfactory in some areas. Presently, a model has been substituted in which the signal is in some ways even less than satisfactory.
A technician can possibly cover 100 sections in a day for the status of cathodic protection. A repair crew may be able to eliminate located grounds in an average of about 1/2 hour. The work of finding each ground in a section comprising 10 blocks or more averages about 2 1/2 hours and can require as much as a week to check thoroughly. Hence, the present state of the art is not satisfactory from the viewpoint of the user.
Firstly, the original or old Wahlquist design, which is marginal at best in ground seeking, has been replaced by a solid state device that is an improvement when used as a pipe and cable tracer, but is not claimed to be effective in tracing grounds.
Secondly, most designs, including the Wahlquist, are subject to various types of interference, particularly 50/60 cycle AC hum, which is usually present in the main and is often stronger than the impressed signal. Furthermore, ignition noise of vehicles acts as a severe interference.
Thirdly, when using instruments, the speed of operation is limited to the speed at which the operator can traverse the ground and yet still attempt to receive signals. This speed is one mile per hour or less with some instruments.